1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an internal combustion engine, and, more particularly, to a secondary air control system in the exhaust gas purifying system which includes a three-way catalyzer in the exhaust passage of the engine and a secondary air feed device adapted for feeding secondary air into the exhaust passage upstream of the catalyzer.
In the exhaust gas purifying system utilizing a catalyzer, the action of the catalyzer depends on the air-fuel ratio or air-fuel equivalence ratio (.lambda.=actual air-fuel ratio/stoichiometric air-fuel ratio); accordingly, it is necessary to control the air-fuel ratio to be within a required range for ensuring an expected effect. Particularly, for a system utilizing a three-way catalyzer, an air-fuel ratio controlled within a small range of around the stoichiometric point (.lambda.=1) is required. Therefore, in the above-mentioned secondary air feed type of the exhaust gas purifying system, the secondary air quantity must be controlled so that, after the feeding of the secondary air, the air-fuel ratio becomes equal to the required air-fuel ratio.
2. Description of the Prior Art
A conventional secondary air control system is of a single feedback type wherein a secondary air control valve is operated in response to an output signal of an oxygen sensor (otherwise called as a lambda sensor) installed on the exhaust passage between the secondary air feeding position and the catalyzer. However, such type of control system has the drawback of not being able to effect a precise control of the secondary air quantity and, accordingly, of the air-fuel ratio, due to the reason mentioned below.
Generally, for feeding secondary air in the exhaust gas purifying system, an air pump driven by the engine is used. A characteristic of the conventional air pump resides in that its delivery flow rate varies linearly in proportion to the engine speed, but such rate does not substantially vary with respect to the engine load. Another characteristic of the air pump resides in that its delivery flow rate is generally set so that a sufficient quantity of secondary air can be obtained when the engine is in a high load operation. Accordingly, the air-fuel ratio varies in accordance with the engine load condition, in such a manner that it is made leaner with the excess secondary air when the engine is in a low load operation. The variation of the air-fuel ratio is detected by the oxygen sensor, and is controlled by the feedback control system. However, this feedback control system has a response lag which is caused by a lag in the transfer of the secondary air from the secondary air feeding position to the oxygen sensor (the lag in the transfer of the secondary air is relatively large during the low load operation of the engine) and by a lag in the operation of the secondary air valve. Accordingly, the air-fuel ratio fluctuates within a relatively large amplitude, and an effective purifying performance thus cannot be achieved.